Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
During some conditions, excessive vacuum can build inside the evaporative emission control system. For example, due to degradation of a canister purge solenoid or a canister vent solenoid, or due to a restriction in the system's fresh air line, fuel tank vacuum levels may become excessive, potentially harming the fuel tank. While evaporative emission control systems may include hardware, such as fuel caps, to relieve excess vacuum, there may be conditions where relief is not provided due to hardware malfunction. Likewise, there may be conditions when mitigating steps taken to address the excess vacuum do not provide the desired level of relief. For example, when the excessive vacuum is due to a stuck open canister purge valve, commanding the purge valve to close may not shut off the vacuum supply from the engine's intake manifold. Consequently, fuel tank vacuum level may continue to rise to dangerous levels. In addition to costly fuel tank repairs and an increase in MIL warranty, this can also result in increased operator dissatisfaction.
The inventors herein have recognized that fuel tank damage can be reduced if the trapped vacuum can be vented as soon as an elevated vacuum level is observed. In one example, this may be achieved by a method for a fuel system coupled to an engine, comprising: in response to fuel tank vacuum level, opening a canister purge valve to dissipate excess fuel tank vacuum into an engine intake manifold while the engine is not combusting, a timing of the opening based on engine operating conditions. In this way, fuel tank vacuum levels can be reliably returned to safer levels.
In one example, during engine running conditions, a fuel tank vacuum may be monitored. In response to a rise in fuel tank vacuum levels (e.g., fuel tank vacuum being higher than a threshold level and/or a rate of rise in fuel tank vacuum level being higher than a threshold rate), it may be determined that fuel tank vacuum needs to be vented to reduce potential fuel tank damage. To relieve the excess vacuum, a canister purge valve may be opened so that the vacuum can be dissipated to the engine intake manifold. A timing of opening of the canister purge valve may be opened based on engine operating conditions. For example, in embodiments where the vehicle is a hybrid vehicle, the canister purge valve may be opened after shifting the vehicle from an engine mode to a battery mode of operation. As another example, the valve may be opened after the vehicle has reached an idle status with the engine not running (e.g., an idle-stop). Alternatively, the fuel tank vacuum may be vented to a combusting engine opportunistically when the purge air inlet pressures are closer to atmospheric pressure levels than the fuel tank. In addition, throttle and fuel adjustments may be concomitantly used to allow a driver torque demand to be met while the vacuum is vented.
In this way, by monitoring changes in vacuum level of an isolated fuel tank, fuel tank vacuum build-up can be detected and addressed before fuel tank degradation is incurred. By venting the excess vacuum to the engine while the engine is not combusting, air-fuel errors are averted. Alternatively, by selectively venting the excess vacuum to the intake while the engine is combusting and when the purge air inlet pressures are closer to atmosphere than the fuel tank, air-fuel errors are reduced. By rapidly and reliably addressing excess fuel tank vacuum, fuel tank degradation can be reduced and fuel system integrity can be better maintained.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.